Research On The Formation Mechanism And Prevention Technology Of Harmful Gas In Hongdoushan Non-coal Tunnel On Dalin Railway

With the improvement of infrastructure construction in our country,many projects show the characteristics of plateau,complicated geological conditions and diversification of geological problems.For example,the number of safety accidents caused by harmful gases is increasing day by day.At present,many harmful gases are found in many tunnels of Dali Lincang Railway.In the construction of inclined shaft No.1 of Hongdoushan Tunnel,a cystic high-pressure gas burst accident occurred.Through field test,it is found that the high pressure gas is mainly high concentration CO2 gas,followed by high concentration H2S gas.By consulting relevant literature,relevant research results in adjacent areas,local chronicles and maps,this paper analyses the formation mechanism of harmful gases in Hongdoushan Tunnel,and puts forward corresponding prevention and control technologies by means of geological survey,summary and analysis of field monitoring data of harmful gases in tunnels,laboratory test analysis and numerical simulation.The research results will have important practical significance for the safety construction of non-coal measures tunnel engineering.The following understandings are obtained:(1)Through long-term on-site monitoring,the main harmful gases in Hongdoushan Tunnel are CO2,H2S,and lower concentrations of CO,H2,SO2,NH3,Cl2,etc.Its impact on tunnel construction is mainly the burst threat of high-pressure gas,and the health and personal hazards of toxic gas to tunnel staff.Harmful gases in Hongdoushan Tunnel are characterized by diversity,multiple hazards(toxicity,flammability and explosiveness),randomness of gas types,uneven distribution and intermittent escape.(2)Carbon isotope identification is the most effective method to identify the source of CO2.The carbon isotope of CO2 in Hongdoushan Tunnel is-7.6‰?-2.5‰,and the carbon isotope of CO2 in hot spring gas is-7.2‰-0.3‰.The content of He in harmful gases in Hongdoushan Tunnel ranges from 5.21?9.81 × 10-6mol/mol.Carbon isotope values indicate that there is CO2 from mantle-magmatic origin.He with deep characteristics also indicates that harmful gases probably come from deep mantle.In addition,the value of ?13C·CO2 in tunnel and hot spring gas is summarized,and there is also CO2 of metamorphic origin.(3)The genetic mechanism of harmful gases can be summarized as follows:mantle-derived materials from deep mantle are transported to shallow strata and surface through deep cutting and long extension of deep and large faults.When encountering a tunnel cavity,harmful gases will escape into the tunnel directly or through secondary faults,joints and fissures to form harmful gases affecting the project.In this process,there are also metamorphic gas components added.In addition,the development of hydrothermal activities proves that the deep circulation of groundwater also communicates the material exchange between the shallow(or surface)and the deep,and accelerates the material migration.(4)The simulation results of stress field and seepage field in inclined shaft No.1 of Hongdoushan Tunnel by COMSOL Multiphysics software show that stress concentration occurs in the middle of vault roof and floor after tunnel excavation.The percolation velocity of harmful gas in surrounding rock decreases gradually with the exposure time of surrounding rock,and with the passage of time,the way of escaping through joints is more dominant than that of fissures.(5)Through TSP detection technology,the geological prediction of No.1 inclined shaft of Hongdoushan Tunnel is carried out in advance,and the harmful gases are predicted according to the prediction results.With the real-time monitoring of construction,the changes of harmful gases in the predicted range are summarized.It is found that TSP detection technology can predict harmful gases in non-coal-bearing tunnels.To do a good job in the prevention and control technology of harmful gases in non-coal-bearing tunnels,the following points should be done:1)to truly understand the harmfulness of harmful gases;2)to use a variety of advanced geological prediction methods in combination;3)to strengthen the management of harmful gas monitoring and ventilation work;4)to maintain a high degree of safety awareness.